A cell-based boundary model of gastrulation by unipolar ingression in the hydrozoan cnidarian Clytia hemisphaerica Maarten van der Sande, Yulia Kraus, Evelyn Houliston, Jaap Kaandorp To cite this version: Maarten van der Sande, Yulia Kraus, Evelyn Houliston, Jaap Kaandorp. A cell-based boundary model of gastrulation by unipolar ingression in the hydrozoan cnidarian Clytia hemisphaerica. Developmental Biology, Elsevier, 2020, 460 (2), pp.176-186. 10.1016/j.ydbio.2019.12.012. hal-02565295 HAL Id: hal-02565295 https://hal.sorbonne-universite.fr/hal-02565295 Submitted on 6 May 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. 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Developmental Biology 460 (2020) 176–186 Contents lists available at ScienceDirect Developmental Biology journal homepage: www.elsevier.com/locate/developmentalbiology A cell-based boundary model of gastrulation by unipolar ingression in the hydrozoan cnidarian Clytia hemisphaerica Maarten van der Sande a, Yulia Kraus b,c, Evelyn Houliston d, Jaap Kaandorp a,* a Computational Science Lab, University of Amsterdam, Science Park 904, 1098XH, Amsterdam, the Netherlands b Department of Evolutionary Biology, Lomonosov Moscow State University, Russia c Koltzov Institute of Developmental Biology of the Russian Academy of Sciences, 26 Vavilov Street, Moscow, 119334, Russia d Sorbonne Universite, CNRS, Laboratoire de Biologie du Developpement de Villefranche-sur-mer (LBDV), 06230, Villefranche-sur-mer, France ARTICLE INFO ABSTRACT Keywords: In Cnidaria, modes of gastrulation to produce the two body layers vary greatly between species. In the hydrozoan Clytia hemisphaerica species Clytia hemisphaerica gastrulation involves unipolar ingression of presumptive endoderm cells from an oral Gastrulation domain of the blastula, followed by migration of these cells to fill the blastocoel with concomitant narrowing of Ingression the gastrula and elongation along the oral-aboral axis. We developed a 2D computational boundary model capable Planar cell polarity of simulating the morphogenetic changes during embryonic development from early blastula stage to the end of Strabismus Cell-based model gastrulation. Cells are modeled as polygons with elastic membranes and cytoplasm, colliding and adhering to other cells, and capable of forming filopodia. With this model we could simulate compaction of the embryo preceding gastrulation, bottle cell formation, ingression, and intercalation between cells of the ingressing pre- sumptive endoderm. We show that embryo elongation is dependent on the number of endodermal cells, low endodermal cell-cell adhesion, and planar cell polarity (PCP). When the strength of PCP is reduced in our model, resultant embryo morphologies closely resemble those reported previously following morpholino-mediated knockdown of the core PCP proteins Strabismus and Frizzled. Based on our results, we postulate that cellular processes of apical constriction, compaction, ingression, and then reduced cell-cell adhesion and mediolateral intercalation in the presumptive endoderm, are required and when combined, sufficient for Clytia gastrulation. 1. Introduction on invagination of the future “oral” side of the blastoderm corresponding to the presumptive endoderm territory (Kraus and Technau, 2006; Magie The process of gastrulation sets up the germ layers and fixes the and Daly, 2007), while in Clytia (¼ Phialidium) species, the endoderm embryonic axes to define the animal body plan. Species of the major forms entirely from cells detaching individually from the blastoderm at animal clade “Bilateria” are triploblastic, and gastrulation creates three the future oral pole and migrating inward to fill the blastocoel, a process germ layers: ectoderm, endoderm and mesoderm. Species in the sister known as unipolar ingression (Byrum, 2001). While Nematostella clade Cnidaria are considered “diploblastic”, i.e. they have only two germ invagination-based gastrulation has been described in detail and simu- layers, ectoderm and endoderm (sometimes termed entoderm). Within lated (Tamulonis et al., 2011), little is yet known about the mechanical both Bilateria and Cnidaria the details of gastrulation vary between basis of hydrozoan gastrulation by unipolar ingression. species, with specific cell populations of the single-layered blastula un- The larva of the hydrozoan model species Clytia hemisphaerica is a dergoing, in different combinations, cell sheet invagination, involution simple two-layered “planula” with a polarized torpedo shape (Houliston and epiboly as well as individual cell delamination and ingression et al., 2010). It swims unidirectionally with its broader “aboral” pole at (Gilbert, 2010). the front, thanks to coordinated beating of single cilia on each ecto- Cnidarian gastrulation includes a rich variety of modes (Kraus and dermal cell. The common orientation of the ectoderm cells and thus of Markov, 2017), of which two extreme modes are seen in the main lab- the cilia is governed by planar cell polarity (PCP) along the aboral-oral oratory model species Nematostella vectensis from Anthozoa and Clytia axis, which involves the segregation of the highly conserved trans- hemisphaerica from Hydrozoa. Nematostella gastrulation is largely based membrane protein Strabismus to the aboral side of each cell, likely * Corresponding author. E-mail address: [email protected] (J. Kaandorp). https://doi.org/10.1016/j.ydbio.2019.12.012 Received 9 July 2019; Received in revised form 9 December 2019; Accepted 21 December 2019 Available online 2 January 2020 0012-1606/© 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). M. van der Sande et al. Developmental Biology 460 (2020) 176–186 interacting with Frizzled 1 protein on the oral side of the adjacent cell blastula. The ingressing cells fill the blastocoel cavity (Fig. 2). As they (Momose et al., 2012). Polarity is established in the egg, and the germ migrate towards the aboral pole, the embryo elongates along the layers are formed during gastrulation. Gastrulation is completed roughly oral-aboral axis. Mid-gastrula stage embryos are roughly pear-shaped, 24 h after fertilization (hpf), see Fig. 1, the exact timing depending on with the oral end being narrower and the aboral end more spherical temperature. Subsequent cell differentiation in both germ layers creates a (Fig. 1 16.5h). metamorphosis-competent planula larva on the third day after fertiliza- At the molecular level we know that the presumptive endoderm ter- tion. We have recently characterized in detail the cell morphological ritory is established by Wnt/Beta-Catenin signalling prior to gastrulation. changes that precede and accompany gastrulation in Clytia (Kraus et al. The location of this territory is determined by the initial polarzed dis- (2019). The period until the end of gastrulation comprises five successive tribution of Wnt and Fz mRNAs in the egg, which define an oral domain and partially overlapping processes: cleavage, epithelialization, centered around the site of the original egg animal pole, and its extent by compaction, cell ingression and elongation. During cleavage, repeated feedback interactions along the developing body axis (Momose and divisions transform the fertilized egg into a monolayered blastula. During Houliston, 2007; Momose et al., 2008). Cells in this domain are fated to epithelialization, the cells of the blastula flatten against each other and become either oral ectoderm or endoderm (Lapebie et al., 2014). The rest become polarized along an apicobasal axis to form an epithelium, with of the cells give rise to lateral and aboral ectoderm (Byrum, 2001). cell-cell junctions developing apically. The compaction process, which The mechanism of embryo elongation during gastrulation in Clytia is occurs in parallel with epithelialization, reduces the overall diameter of not fully understood, but it is likely to involve intercalation between cells the embryo while the blastoderm layer increases in thickness. Gastrula- perpendicular to the oral-aboral axis, in a similar manner to elongation of tion starts at about 11–12 hpf. It involves individual ingression of pre- chordate embryos by intercalation of involuting mesoderm cells sumptive endoderm cells into the blastocoel from a domain covering (convergent extension) (Wallingford et al., 2002). Supporting this idea, about one third of the blastoderm, centered on the oral pole of the patches of marked cells in lateral regions become elongated along the Fig. 1. Clytia embryogenesis from egg to the end of gastrulation. DIC images of embryos at successive times post fertilization from a population developing syn- chronously at 18 ∘C are shown. Note that the overall morphology of blastula and gastrula stage embryos is quite irregular and can vary considerably between embryos. After the initial cleavage divisions, cells organize into an epithelial sheet and the embryo becomes more compact as each cell polarizes and elongates along its apicobasal axis. Gastrulation starts at around 12